Amplifier circuit

ABSTRACT

An amplifier circuit includes a circuit path of serially connected complementary type transistors. First and second feedback loops include a loop amplifier, the transistors of the circuit path and a corresponding resistor.

TECHNICAL FIELD

The present disclosure relates to an amplifier circuit. Specifically,the present disclosure relates to a low noise amplifier circuitincluding input transistor, resistor network and feedback loop. Theamplifier circuit may be used to amplify signals generated by a sensor.

BACKGROUND

Amplifier circuits are widely used in electronic devices to amplify theamplitude of an input signal so that the amplified output signal can befurther processed in analog and digital signal processing circuitry. Theto be amplified signal may be very small such as a signal obtained froma sensor so that the amplifier should have a noise level that is as lowas possible. Conventional low noise amplifiers (LNAs) include a feedbackloop to ensure high accuracy, high linearity and low distortion levels.For a sensor generated input signal, the input impedance of theamplifier should be relatively high or infinite. Although a low noiselevel may require additional circuit components, the power consumptionof the amplifier should be low.

It is an object of the present disclosure to provide an amplifiercircuit having high accuracy, high linearity and low noise levels.

It is another object of the present disclosure to provide a sensorarrangement including such an amplifier.

SUMMARY

One or more of the above-mentioned objects are achieved by an amplifiercircuit comprising the features of present claim 1.

An amplifier circuit according to the present disclosure comprises acircuit path that includes first and second serially coupled transistorsof complementary conduction type such as a series connection of thedrain source paths of a PMOS and a NMOS transistor. The to be amplifiedsignal is supplied to the control terminals of the first and secondtransistors. Feedback loops include the first and second transistors anda first and a second resistor connected to output terminals of theamplifier circuit. The feedback loops include a loop amplifier havingcomplementary output terminals of which the input is connected to a nodebetween the first and second transistors of the circuit path. Third andfourth resistors are connected to the first and second resistors of thefeedback loops to form respective defined ohmic voltage dividers thatprovide an amplification factor given by the relation of the resistors.The third and fourth resistors may be shared with a complementaryoperating circuit that enables differential input and output signals.For single-ended operation, the third and fourth resistors may also becoupled to reference potentials.

The first and second complementary transistors include in first andsecond feedback loops in connection with the resistor network allows aLNA amplifier with low power consumption and 3 dB noise improvement.

First and second differential pairs of transistors may be connected tothe first and second transistors of the circuit path to obtain a stackedarchitecture. The differential pair transistors are operated with acomplementary input signals so that the first and second transistors andone of the differential pair transistors receive the same input signal.The stacked architecture increases the signal-to-noise ratio up to 6 dB.

The differential pair transistors of the stacked variant should exhibita lower threshold voltage compared to the first and second transistorsof the circuit path. Low voltage threshold (LVT) transistors can beachieved by various measures known to a skilled person such asincreasing the gate oxide thickness, additional doping or implants atthe channel region or additional voltages at the bulk region. Othermeasures apparent to a skilled circuit designer are also useful.Conventional and LVT transistors in the stacked arrangement achieveproper working points of the transistors to ensure linearity andaccuracy of signal amplification.

A current conveyer circuit may be connected from the node between thefirst and second transistors of the current path to the resistor networkor to the outer terminals of the current path such as the sourceterminals of the first and second transistors. The current conveyercircuit provides a correction current to the current path transistors sothat linearity of the amplifier is increased and thermal dependenciesare reduced. The LNA circuit including the current conveyer circuitprovides an increased signal-to-noise level at high linearity over awide temperature range.

The current conveyer circuit includes circuitry that senses the currentthrough the circuit path and provides a replica current to the outerterminals of the circuit path which is connected to the resistornetwork. Specifically, the current conveyer circuit may comprise a pairof source connected push pull transistors to sense the current throughthe circuit path wherein a drain terminal of the push pull transistorsprovides the replica current output. Bias currents may be provided tothe source connected transistors through corresponding current mirrorcircuits. Additional replica current outputs can be provided throughcurrent mirrors.

According to an embodiment of the current conveyer circuit, a feedbackloop through a comparator amplifier may control the current through areference current path that generates reference potentials that aresupplied to the source connected push pull transistors. The comparatoramplifier receives also a common mode voltage as a reference. The commonmode voltage may be substantially in the middle of the signal voltageswing. The feedback loop including comparator amplifier decreases theinput impedance. The source connected transistors have the function of apush pull current sensor.

According to one embodiment, the current conveyer circuit may be anon-inverting circuit that senses the current in the current path andprovides the replica current to that current path. According to anotherembodiment, the current conveyer circuit may be an inverting circuitthat senses the current in the current path at one side of acomplementary amplifier topology and provides the replica current to theother, complementary circuit side. In the non-inverting current conveyercircuit, sensing and replica output current is performed within the samepush pull stage. In the inverting current conveyer circuit, sensing andreplica output current is performed within the different current pathsconnected through current mirrors.

The loop amplifier can be realized with different circuits. According toone embodiment, the loop amplifier comprises a differential amplifier ofwhich the input is connected to the node between the first and secondtransistors of the circuit path. The differential loop amplifiergenerates an output signal to control corresponding transistors that areconnected to the resistor network, specifically to control the currentthrough the first and second resistors. A buffer transistor transfersthe potential from the node between the first and second transistors ofthe circuit path to the input of the differential loop amplifier.

According to another embodiment, the loop amplifier may comprise a classAB stage including first and second transistors that are connected tothe resistor network. Specifically, the class AB transistors areconnected in series with the first and second resistors. A buffertransistor is connected to the node between the first and secondtransistors of the circuit path and to one of the transistors of theclass AB stage. Complementary transistors are provided for biasing thecontrol terminals of the transistors of the class AB stage.

As an alternative to a current conveyer circuit for linearitycorrection, a fifth and a sixth resistor may be provided connectedbetween the differential pair transistors. An additional transistor maybe provided to connect the fifth and sixth resistor with the first andsecond resistors, respectively, so that the input stage of the amplifierhas a symmetrical shape. A correction current is injected at theterminals of the fifth and sixth resistors. The correction current isgenerated by additional class AB stages controlled by the node betweenthe first and second transistors of the current path. This embodimentmay be preferably operated with a class AB stage loop amplifier thatprovides the output transistors connected serially with the first andsecond resistors.

According to an embodiment, the fifth and sixth resistors may have twicethe size of the other resistors such as the first and second resistor.The class AB stage output transistors connected to the resistor networkmay have transistors of twice the size than the transistors of theadditional class AB stages that are connected to the terminals of thefifth and sixth resistors.

A low noise amplifier according to the above-described principles may beused to amplify signals of low amplitude such as signals from a sensor.The sensor signal may be a differential signal that depends on anambient condition determined by the sensor. The differential sensorsignal may be very weak and close to noise level so that the LNAamplifies the signal and increases the signal-to-noise ratio at highlinearity and low power consumption.

It is to be understood that both the foregoing general description andthe following detailed description are merely exemplary, and areintended to provide an overview or framework to understand the natureand character of the claims. The accompanying drawings are included toprovide a further understanding and are incorporated in, and constitutea part of, this description. The drawings illustrate one or moreembodiments, and together with the description serve to explainprinciples and operation of the various embodiments. The same elementsin different figures of the drawings are denoted by the same referencesigns.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 shows a conventional low noise amplifier;

FIG. 2 shows another conventional low noise amplifier;

FIG. 3 shows a low noise amplifier according to the principles of thepresent disclosure;

FIG. 4 shows a low noise amplifier according to the principles of thepresent disclosure including stacked input transistors and currentconveyer circuit;

FIG. 5 shows a schematic representation of a portion of a low noiseamplifier;

FIG. 6 shows a non-inverting current conveyer circuit to be used inconnection with the circuit of FIG. 5;

FIG. 7 shows an inverting current conveyer circuit to be used inconnection with the circuit of FIG. 5;

FIG. 8 shows a schematic representation of another low noise amplifier;and

FIG. 9 shows a block diagram of a sensor circuit including a low noiseamplifier according to the principles of the present disclosure.

DETAILED DESCRIPTION OF EMBODIMENTS

The present disclosure will now be described more fully hereinafter withreference to the accompanying drawings showing embodiments of thedisclosure. The disclosure may, however, be embodied in many differentforms and should not be construed as limited to the embodiments setforth herein. Rather, these embodiments are provided so that thedisclosure will fully convey the scope of the disclosure to thoseskilled in the art. The drawings are not necessarily drawn to scale butare configured to clearly illustrate the disclosure.

FIG. 1 shows a conventional instrumentation amplifier. The amplifierincludes a PMOS input transistor M1 receiving an input signal inp.Transistor M1 is supplied with a constant current by PMOS transistorMpout through resistor 101 connected in series with transistor M1. Atransistor Rin is connected to the source of transistor M1. The currentthrough M1 is drained through a constant current source 102 connected tothe drain of transistor M1 draining current Ib+Ic. A feedback loopcomprises NMOS folding transistor Mfold controlled by a constantpotential Vb and constant current source Ic connected to the high side.The loop of folding transistor Mfold and output transistor Mpoutprovides a constant current through input transistor M1. The currentsource Ic and current 102 supplying current Ib+Ic control the workingpoints of the transistors. The source terminal of transistor M1 directlyfollows the input signal inp. The voltage divider between resistor 101and resistor Rin allows an amplification factor so that the signal atthe drain of transistor Mpout is an amplified signal of the input signalinp. The circuit of FIG. 1 comprises left-hand and right-hand portionsthat have identical structures, however, are controlled by thecomplementary input signals inp, inn, wherein both complementarysections are coupled across resistor Rin. Alternatively, also asingle-ended circuit is conceivable wherein one end of resistor Rin maybe connected to a reference potential or ground.

The differential input signals inp, inn are transferred unaltered to thesources of the input transistors M1, M2. The signal current generatedacross resistor Rin is supplied by the output stages Mpout of thefeedback loops. A differential output signal is supplied at the drainterminals of the output transistors Mpout that is connected to theresistors 101. The two oppositely disposed level shift transistors M1,M2 receive the differential input signal inp, inn and drive the resistorRin between their sources. Both level shift transistors M1, M2 areincluded in an associated feedback loop to transfer the input signals tothe resistor terminals with accuracy. The two load resistors 101 collectthe generated current and provide the amplified output signal. Thecurrent through transistors M1, M2 is substantially constant so that theinput signal is transferred unaltered to the terminals of the resistorRin. The output devices Mpout supply the signal current across resistorRin so that the current flow across the series load resistors 101provides an amplification of the input signal inp, inn.

Turning now to FIG. 2, another conventional low noise amplifier isshown. The circuit in FIG. 2 includes, firstly, differentially operatedleft and right portions and, secondly, complementary upper and lowerportions operated with the same input signal. The circuit includes NMOSinput transistor Mn1 and PMOS input transistor Mp1 that are seriallyconnected and controlled by the input signal inp to provide an outputsignal outn at the output terminal connected to the node between Mp1 andMn1. The circuit path of transistors Mn1, Mp1 is supplied by identicalcurrents Itail connected between the transistors Mp1 and Mn1 and supplyand ground potentials, respectively. The complementary upper and lowerportions of the amplifier circuit of FIG. 2 operate in parallel and arebiased by a current generator Itail. The overall transconductance isdoubled while the voltage noise increases only with factor 1.4 becauseof the quadratic sum. This leads to a 3 dB reduction of input referrednoise. The output currents at the output signals outn, outp arecollected by mirrors or current buffers to drive an output stage of theamplifier (not shown in FIG. 2).

FIG. 3 shows a principle diagram of a low noise amplifier (LNA) circuitaccording to the principles of the present disclosure. The LNA circuitcomprises a left-hand portion and a right-hand portion allowingdifferential processing of the complementary input signals inp, innconstituting a differential type input signal. Left-hand and right-handportions have identical structures operated by complementary signals.The upper and lower portions have complementary structures and areoperated by the same input signals.

A circuit path 320 includes PMOS input transistor Mp1 and NMOS inputtransistor Mn1 of which the drain source paths are serially connectedforming current path 320. The input terminals of transistors Mp1, Mn1are connected to input terminal 321 receiving input signal inp. Afeedback loop to PMOS transistor Mp1 includes a loop amplifier 301 and afeedback resistor 302 connected from the output op1 to the source oftransistor Mp1. Another feedback loop to NMOS transistor Mn1 includes aloop amplifier 301 connected from another output op2 through resistor303 to the source of transistor Mn1. The loop amplifier 301 has an inputthat is connected to the node 306 disposed between input transistorsMp1, Mn1. Resistors 304, 305 are connected to the resistors 302, 303 andthe sources of transistors Mp1, Mn1, respectively. The resistor networkof resistors 302, 304 and 303, 305 each form a divider such as a voltagedivider such as an ohmic voltage divider so that the output signals atterminals op1, op2 are amplified with regard to the input signal inp atinput terminal 321. The amplification factor depends on or is determinedby the ratio between resistors 302, 304 or 303, 305. Resistors 304, 305are shared between the left-hand and right-hand differentially operatedcircuit portions. In a single-ended version, resistors 304, 305 may beconnected to a reference or ground potential. Resistors 302 and 304 forman ohmic voltage divider, and resistors 304 and 305 form another ohmicvoltage divider. The ohmic voltage divider and the other ohmic voltagedivider may each include a pair of resistors, wherein the resistor pairsare corresponding or the same. The current generated across resistor 304is forced to cross resistor 302 by the arrangement made up by transistorMP1 and amplifier 301 so that resistors 302, 304 act as a voltagedivider as the resistors are crossed by the same current. Resistors 303,305 exhibit the same function.

The feedback loop of the amplifier of FIG. 3 comprises the inputtransistors Mp1, Mn1. The input to the feedback loop is at the coupleddrains of the input transistors. The output of the feedback amplifierhas two branches, one for pullup and the other one for pulldown. Theinput stage requires two resistors 304, 305 of equal resistance Rin. Theoutput signals at output terminals op1, op2 are the currents through theload resistors 302, 303 and are to be summed up with a convenientsumming operation. The loop amplifier 301 uses a virtual ground set at asuitable voltage to comply with the dynamic range of the inputtransistors Mp1, Mn1. The input signal inp is buffered by the levelshift transistors Mp1, Mn1 and transferred across the input resistors304, 305. The output stage of the loop amplifier 301 sinks or sourcesthe generated currents. With the load resistors 302, 303, the outputvoltage at terminals op1, op2 can be conveniently summed with thecorrect sign. In case of a differentially operating circuit usingleft-hand and right-hand portions, the voltages at terminals op1, op2,on1, on2 must be summed conveniently with the correct sign. The circuitshown in FIG. 3 achieves a noise improvement of 3 dB through thecomplementary arrangement of complementary PMOS and NMOS inputtransistors Mp1, Mn1. Additional improvements may be useful as explainedhereinbelow to address thermal drift of the gain and signal distortion.

FIG. 4 shows an expanded version of the circuit of FIG. 3. The circuitpath 320 shows a stacked architecture including complementary additionaldifferential transistor pairs PMOS transistors Mp2 sp, Mp1 sp havingsource terminals connected with each other and connected to the drainterminal of PMOS transistor Mp1. Further included in circuit path 320 isa differential pair of NMOS transistors Mn2 sp, Mn1 sp having sourceterminals connected with each other and connected to the drain of NMOStransistor Mn1. The control terminals of transistors Mp1, Mn1 and Mp1 spand Mn1 sp are connected to the input terminal 321 receiving inputsignal inp. The control terminals of the other transistors of thedifferential transistor pairs Mp2 sp and Mn2 sp are connected to inputterminal 331 carrying complementary input signal inn. The transistors ofthe differential transistor pairs are low voltage threshold (LVT)devices that may be obtained by a thicker gate oxide, a special channeldoping or a bulk bias voltag or a combination of said measures. Theright-hand portion of the circuit has an identical structure, however,the control terminals of the input transistors are supplied with thecomplementary input signals to allow differential operation of thecircuit. The stacked implementation has double transconductance of theinput transistors while the noise is quadratically summed which isanother 3 dB improvement in noise achievement. The overall performancemakes a 6 dB noise improvement while maintaining the same powerconsumption.

The drain terminals of the differential transistor pairs are connectedto each other to reuse the current from one of the differentialtransistor pairs such as transistors Mp2 sp, Mp1 sp in the other one ofthe differential transistor pairs such as transistors Mn2 sp, Mn1 sp, asshown in FIG. 4. Alternatively, it is also possible to disconnect thedifferential transistor pairs and connect one of the transistors of adifferential transistor pair such as transistor Mp2 sp to groundpotential and one of the transistors of the other differentialtransistor pair such as transistor Mn2 sp to supply potential (not shownin FIG. 4). The latter transistors are those that are not connected tothe loop amplifier. While transistors Mp2 sp, Mn2 sp connected to groundand reference potential, respectively, have an increased powerconsumption, the circuit shown in FIG. 4 with the shared drains of Mp2sp, Mn2 sp has reduced power consumption.

A current conveyer circuit 410 is provided to sense the current at thedrain terminals of the transistors of the differential transistor pairsthat are not connected to the loop amplifier 301. Specifically, thedrain terminals of transistors Mp2 sp, Mn2 sp are connected to the input411 of current conveyer circuit 410. Current conveyer circuit 410comprises two output terminals 412, 413 that are connected to thesources of the transistors Mp1, Mn1. The outputs 412, 413 of the currentconveyer circuit 410 are at the same way connected to the node betweenthe resistors 302, 304 and 303, 305 which are, in turn, connected to thesources of the transistors Mp1, Mn1. The current conveyer circuit 410generates a replica from the current sensed at terminal 411 and forwardsthe replica currents at terminals 412, 413 to the sources of transistorsMp1, Mn1 of circuit path 320. The current conveyer injects a currentinto the circuit path 320 to enhance linearity and avoid a thermal driftissue. The replica currents generated by current conveyer 410 compensatethe effect of the currents through the transistors Mp2 sp, Mn2 sp tocorrect the current contribution by these transistors through theresistors 302, 303.

FIG. 5 in connection with FIG. 6 or 7 show a detailed schematic diagramof the circuit of FIG. 4. It is to be noted that the left-hand portionof FIG. 4 is shown at the right-hand side of FIG. 5. FIG. 5 includes thestacked transistor arrangement along the circuit path 320. The loopamplifier 301 is represented in FIG. 5 on transistor level. Amplifier301 includes a differential pair of transistors Ma1, Ma2 supplied by aconstant current Itail. The control terminal of transistor Ma1 iscontrolled by the input 306 of the loop amplifier connected to thecoupled drain terminals of transistors Mplsp, Mnlsp of the current path320. A common gate buffer transistor Mnc provides a signal level shiftfrom node 306 to transistor Ma1. The buffer transistor Mnc is controlledby a bias voltage Vbias, the source of buffer transistor Mnc isconnected to node 306 and the drain of buffer transistor Mnc isconnected to the control terminal of transistor Ma1 of the differentialamplifier 301. The drain source path of buffer transistor Mnc includes acorresponding constant current source Ibc at the high side and the lowside. The buffer transistor Mnc prevents a large swing at the loopamplifier input from appearing also on node 306 which might affectlinearity so that transistor Mnc improves linearity of the amplifier.Transistor Mnc is not mandatory and may be omitted or replaced by otherelements. The output transistor Mpout connected to the load resistor 302is controlled through current mirror 311 of amplifier 301. In acorresponding way, the complementary output transistor Mnout connectedto resistor 303 is controlled by amplifier 301 through current mirrors312, 313. The left-hand portion 550 of the circuit has the samestructure, however, operated with the complementary input signals toachieve a fully differential operating scheme.

The LNA has input transistors Mp1, Mn1 embedded in a feedback loop. Theinput signal inp is transferred without appreciable distortion ontoresistors 304, 305. The signal current generated in this way comes fromthe feedback loop output stage Mpout, Mnout to cross the resistors 302,303, wherein resistors 302, 303 are matched to resistors 304, 305, resp.In this way, an amplified version of the input signal is available atthe terminals op1, op2 of the resistors 302, 303. With resistors 304,305 having a resistance value of R1 and resistors 302, 303 having aresistance value of R2, at the terminals of resistors 302, 303, anamplified version of the input signal inp is available, which is 1+R2/R1times bigger than the input signal. The resistance values R1 and R2 maybe different. In another variant, the resistance values R1 and R2 may bethe same or substantially the same. Noise is substantially dictated bythe input transistors Mp1, Mn2 and the resistors 304, 305. The outputvoltage of the fully differential implementation is the sum of thevoltage drops across the four load resistors 302, 303 and thecorresponding resistors in circuit portion 550.

In FIGS. 6 and 7, two alternative versions of current conveyer circuitsare shown that form part of the circuit of FIG. 5. For reasons ofsimplicity, these circuits are moved to separate figures. FIG. 6 relatesto a non-inverting version and FIG. 7 relates to an inverting version ofthe current conveyer circuits that may be used alternatively.

The non-inverting current conveyer circuit 410 of FIG. 6 comprises inputtransistors minn, minp. Transistors minn, minp are connected in pushpull fashion having current source transistors at the high and lowsides. The inputs of circuit 410 are the shared drains of Mp2 sp, Mn2 spcarrying signal Vcm_p. One output of current conveyer 410 is the drainof push pull transistor minn which supplies a replica current to thesource of transistor Mp1 of the current path 320 and/or the node betweenresistors 302, 304. In operation, circuit 410 senses the current is atthe shared drain of transistors Mp2 sp, Mn2 sp and outputs a replicacurrent is/2 to be forwarded to the drain of transistor Mp1 labelled asnode SPH. Complementary-wise, another replica current is/2 is to besupplied to the source of transistor Mn1 at node SPL. This current isgenerated in a corresponding additional current path in current conveyer410 including push pull transistors minn2, minp2. Additional currentpaths including several current mirrors are used in circuit 410 toprovide proper biasing of the currents. Current conveyer circuit 420 isconnected to the left-hand circuit portion 550 and senses signal Vcm_nat the shared drains between transistors Mp2 sn, Mn2 sp and supplies arespective replica current to the sources at transistors Mp2, Mn2 atnodes SNH, SN1. The currents across the LVT transistors which are notembedded in the feedback loop, for example, the currents throughtransistors Mp2 sp, Mn2 sp may be a source of relevant distortion. Thecurrent conveyer senses these currents and injects replics at thesources of the input devices Mp1, Mn1, to prevent them from crossing theload resistors 302, 303 to contribute to the output voltage. Thisminimizes the overall LNA distortion.

According to an embodiment, the input signal Vcm_p from the shareddrains of Mp2 sp, Mn2 sp is forwarded to a comparator amplifier 601 alsosupplied with a common mode voltage Vcm. The output of the comparatoramplifier 601 controls a current path with push pull bias transistorsMb, Ma which control the push pull control transistors minn, minp,minn2, minp2. This avoids an excessive voltage swing at the input stagewhen a large current is injected into the input of the current conveyerwhich might generate a source of distortion. This is avoided with theminimized input impedances of the comparator amplifier 601. The feedbackloop in the current conveyer prevents large swing at the input of thecurrent conveyer from coupling into the input stage thereby avoidingdistortion. The common sources of the push pull source connectedtransistors minn, minn2, minp, minp2 are regulated by opamp 601 at aconvenient reference voltage Vcm regardless of the amount of currentthat the regulator should sink or source.

FIG. 7 shows an alternative current conveyer circuit of the invertingoperating type. Circuit 710 includes push pull input transistors minn,minp which are connected to the shared drains of transistors Mp2 sn, Mn2sn in the left-hand portion 550 of the circuit of FIG. 5. The outputterminals of circuit 710 are provided by push pull stages 711, 712 thatform current mirrors with the input current path including transistorsminn, minp. The output stage 711 of current conveyer 710 including nodeSPH is connected to the source of transistor Mp1 of circuit path 320 atthe right-hand portion of the circuit of FIG. 5. Correspondingly, theoutput node SPL of output stage 712 is connected to the source oftransistor Mn1 of FIG. 5. The input of current conveyer circuit 720 issupplied with signal Vcm_p at the right-hand portion of the circuit ofFIG. 5 and supplies the output signals to nodes SNH, SNL at theleft-hand portion 550 of FIG. 5. In a fully differential amplifiersolution it is a matter to swap the injection points towards thecomplementary section.

The current conveyer circuits of FIGS. 6 and 7 suppress the main sourcesof distortion and thermal drift in FIG. 5 by supplying replica currents.The non-inverting version of FIG. 6 reduces the power consumption ashalf of the signal current goes directly from the input to the outputwithout crossing any mirror, avoiding load to any supply rail. Theoutput of the non-inverting conveyer of FIG. 6 is connected to the sameright or left-hand section where the input belongs. The invertingconveyer circuit of FIG. 7 senses the current from one section andsupplies the output current replicas to the complementary section. Theinverting version offers more flexibility because the output currentsalways require a current mirror which also allow a multiplication of thecurrent. The inverted version of FIG. 7 allows also a feedback loop withcomparator amplifier 701 to minimize the impedance at its input.

Turning now to FIG. 8, another realization of a low noise amplifier isshown which is alternative to the circuits in FIGS. 5 to 7. The circuitof FIG. 8 uses a class AB type loop amplifier instead of a differentialtype amplifier and uses an alternative to a current conveyer circuit toimprove linearity of the amplifier. In more detail, the circuit in FIG.8 includes an additional resistor 802 that is connected between thesources of the differential pair transistors Mp2 sp, Mp1 sp.Correspondingly, resistor 803 is connected between the drains of thecomplementary differential pair transistors Mn2 sp, Mn1 sp. A PMOStransistor 804 is furthermore connected between the resistors 802 and302 or between the source of transistor Mp2 sp and the resistor 302connected to the output transistor Mpout at output terminal op1.Transistor 804 is controlled by input signal inp, the same as transistorMp1 of current path 320. Correspondingly, transistor 805 is connectedbetween resistors 803, 303 or between the source of transistor Mn2 spand the resistor 303 connected to the output transistor Mnout at outputterminal op2 in the complementary lower portion of the circuit.Additional resistor 802 and additional transistor 804 as well asadditional resistor 803 and additional transistor 805 inject anadditional current into transistors Mp2 sp and Mn2 sp, respectively,which increases the linearity of the amplifier stage.

According to embodiments, the resistance values of the resistors 302 and304 have a ratio on which the gain depends. The additional resistor 802has two times the resistance value of resistor 304. Correspondingly, theresistance values of the resistors 303 and 305 have the same ratio,wherein the additional resistor 803 has two times the resistance valueof resistor 305. Furthermore, resistors 302 and 303 have the same valueand resistors 304 and 305 have the same value.

Concerning the loop amplifier, a class AB type amplifier is used in thecircuit of FIG. 8. The class AB type amplifier comprises a class ABstage of output transistors Mpout, Mnout connected to resistors 302,303. Furthermore, additional class AB stages such as class AB transistorstage 801 a, 801 b are provided having an output terminal p1 which isconnected to one terminal of the additional resistor 802. Anotherterminal of resistor 802 is connected to terminal n1 of a complementaryoperated class AB stage 810 a, 810 b disposed in the right-hand portionof the circuit. Each one of the two terminals of resistors 802, 803 iscontrolled by a class AB stage. An array of replica transistors Mprep,Mnrep is provided on either left-hand and right-hand portion of thecircuit supplying correction currents to the terminals of the resistors802, 803.

The class AB stage receives an input signal from node 806 which is theshared drain of the transistors Mp2 sp, Mn2 sp. Node 806 is connected tothe source of a buffer transistor Mc of which the drain is connected tothe class AB stages such as 801 a, 801 b. The class AB amplifierfurthermore comprises biasing transistors Mfp, Mfn of complementary typesupplied by corresponding bias signals Vp, Vn and connected between thecontrol terminals of the transistors of the class AB stages such as thegate terminals of transistors 801 a, 801 b. The bias potential Vp, Vnare generated at diode stages 820, 821. Comparing the class AB loopamplifier of the circuit of FIG. 8 with the differential loop amplifierof the circuit of FIG. 5, the class AB arrangement can sustain a largerinput voltage value at the expense of a larger power consumption.

The output transistors Mpout, Mnout have twice the size of the class ABstage transistors such as 801, 801 b of the replica array. The circuitof FIG. 8 includes an implementation of a class AB output stage drivearray for the loop amplifier and additional resistors at each inputdifferential pair source. Matched replicas of class AB output stageinject a signal replica onto each terminal of the resistors 802, 803. Aclass AB stage keeps current consumption low and provides a large signalrange at the LNA input. Concerning the values for the resistors, it issufficient that all the products of R*is, with is being the signalcurrent injected at the terminals of the resistors by the output stagereplicas, are equal in all the input pair associated resistors. Thismakes a degree of freedom on the value of R and pursues a trade-offbetween noise and power consumption versus the input signal.

Turning now to FIG. 9 a sensor arrangement is shown including a sensor901 that generates a differential output signal inn, inp of lowamplitude in a noisy circuit environment. The differential output signalof sensor 901 is forwarded to differential low noise amplifier 902 whichgenerates a differential output signal outn, outp with high linearityand reduced noise level. Practically, the noise improvement is about 6dB.

It will be apparent to those skilled in the art that variousmodifications and variations can be made without departing from thespirit or scope of the disclosure as laid down in the appended claims.Since modifications, combinations, sub-combinations and variations ofthe disclosed embodiments incorporating the spirit and substance of thedisclosure may occur to the persons skilled in the art, the disclosureshould be construed to include everything within the scope of theappended claims.

1. An amplifier circuit, comprising: a circuit path including a firsttransistor and a second transistor, the first and second transistorscoupled in series and being of complementary type; an input terminaloperable to receive an input signal, the input terminal connected tocontrol terminals of the first and second transistors; a loop amplifierhaving an input terminal connected to a node disposed between the firstand second transistors and having a first and a second complementaryoutput terminal; a first feedback loop including the first transistor,the loop amplifier and a first resistor connected to the first outputterminal of the loop amplifier; a second feedback loop including thesecond transistor, the loop amplifier and a second resistor connected tothe second output terminal of the loop amplifier; a third resistorconnected to the first resistor and a fourth resistor connected to thesecond resistor.
 2. The amplifier circuit according to claim 1, whereinthe first and third resistors and the second and fourth resistors eachform a voltage divider.
 3. The amplifier circuit according to claim 1,further comprising: a first differential pair of transistors, of whichone of the transistors is included in the circuit path and connected inseries with the first transistor, a second differential pair oftransistors, of which one of the transistors is included in the circuitpath and connected in series with the second transistor, wherein thecontrol terminals of the first and second transistors and the one of thetransistors of the first and second differential pair of transistors areconnected to the input terminal and the control terminals of the othertransistors of the first and second differential pairs of transistorsare connected to another input terminal operable to receive anotherinput signal which is complementary to the input signal.
 4. Theamplifier circuit according to claim 3, wherein the transistors of thefirst and second pair of transistors are configured as low thresholdvoltage transistors having a threshold voltage lower than the thresholdvoltage of the first and second transistors.
 5. The amplifier circuitaccording to claim 4, further comprising a current conveyer circuithaving an input terminal connected to the other transistors of the firstand second differential pair of transistors and having a first outputterminal connected to the first transistor of the current path or to anode between the first and third resistors and having a second outputterminal connected to the second transistor of the current path or to anode between the second and fourth resistors, the current conveyercircuit configured to sense the current at its input terminal andgenerate replicas of the current at its first and second outputterminals.
 6. The amplifier circuit according to claim 5, the currentconveyer circuit comprising at least one pair of source connectedtransistors, of which a node coupling the source terminals of the sourceconnected transistors is connected to the input terminal of the currentconveyer circuit and of which a drain terminal of one of the sourceconnected transistors is connected to one of the first and second outputterminals of the current conveyer circuit.
 7. The amplifier circuitaccording to claim 5, the current conveyer circuit further comprising afeedback loop including a comparator amplifier that is connected to theinput terminal of the current conveyer circuit and to a terminal for acommon mode voltage, the comparator amplifier controlling a referencepotential path that supplies reference potentials to the pair of sourceconnected transistors.
 8. The amplifier circuit according to claim 3,further comprising a complementary operating circuit portion connectedto the third and fourth resistors.
 9. The amplifier circuit according toclaim 8, further comprising another current conveyer circuit having aninput terminal connected to the differentially operating circuit portionand having a first output terminal connected to the node between thefirst and third resistors and having a second output terminal connectedto the node between the second and fourth resistors, the other currentconveyer circuit configured to sense the current at its input terminaland generate replicas of the current at its first and second outputterminals.
 10. The amplifier circuit according to claim 3, wherein theloop amplifier comprises a differential amplifier having an inputterminal controlled by a circuit node coupled between the first andsecond transistors of the circuit path, and having an output terminalcontrolling a transistor connected in series with the first resistor andhaving another output terminal controlling a transistor connected inseries with the second resistor.
 11. The amplifier circuit according toclaim 10, further comprising a buffer transistor having a controlterminal connected to a bias potential and having a source terminalconnected to the node coupled between the first and second transistorsof the circuit path and having a drain terminal connected to the inputof the differential amplifier, the drain and source terminals of thebuffer transistor connected to current sources.
 12. The amplifiercircuit according to 3, wherein the loop amplifier comprises a class ABstage, the class AB stage including: a first transistor connected inseries with the first resistor and a second transistor connected inseries with the second resistor; a buffer transistor having a sourceterminal connected to a circuit node coupled between the first andsecond transistors of the circuit path and having a drain terminalconnected to the control terminal of one of the first and secondtransistors of the class AB stage; and complementary transistorsconnected between the control terminals of the first and secondtransistors, the complementary transistors controlled by referencepotentials.
 13. The amplifier circuit according to claim 12, comprisinga fifth resistor connected between the first differential pair oftransistors and a transistor connected between the fifth and firstresistors and a sixth resistor connected between the second differentialpair of transistors and another transistor connected between the sixthand second resistors wherein an additional class AB stage is connectedto each one of the terminals of the fifth and sixth resistors.
 14. Theamplifier circuit according to claim 13, wherein the fifth and sixthresistors have a resistance value of twice the resistance value of thethird and fourth resistors and wherein the first and second transistorsof the class AB stage have twice the size of the transistors of theadditional class AB stages.
 15. A sensor arrangement, comprising: asensor configured to provide a differential signal dependent on anambient condition; and an amplifier circuit comprising: a circuit pathincluding a first transistor and a second transistor, the first andsecond transistors coupled in series and being of complementary type; aninput terminal operable to receive an input signal, the input terminalconnected to control terminals of the first and second transistors; aloop amplifier having an input terminal connected to a node disposedbetween the first and second transistors and having a first and a secondcomplementary output terminal; a first feedback loop including the firsttransistor, the loop amplifier and a first resistor connected to thefirst output terminal of the loop amplifier; a second feedback loopincluding the second transistor, the loop amplifier and a secondresistor connected to the second output terminal of the loop amplifier;a third resistor connected to the first resistor and a fourth resistorconnected to the second resistor, wherein the amplifier circuit isconfigured to receive the differential signal and configured to outputan amplified signal of reduced noise.
 16. The amplifier circuitaccording to claim 2, wherein the voltage divider of the first and thirdresistors and the voltage divider of the second and fourth resistors areohmic voltage dividers.
 17. The amplifier circuit according to claim 1,wherein the first and third resistors are connected to a source terminalof the first transistor and the second and fourth resistors areconnected to a source terminal of the second transistor.